For years there has been a demand for a simple, accurate, and inexpensive device for measuring and controlling the delivery of intravenous solutions. Traditionally, IV solution flow rates have been set by timing the number of observed drops generated in a drip chamber. On the premise that drop volume is constant, and does not vary significantly from fluid to fluid, delivery rates may be readily calculated.
The problem, which is well recognized, is that drop volume is really not a constant but depends on numerous variables, with the result that the difference between actual flow rates and calculated flow rates may be substantial, conceivably as much as 30 percent or more. Since intravenous solutions frequently contain drug additives, it is apparent that variations in drop sizes, or the inability to determine accurately and quickly the precise volume of a drop, may result in substantial dosage errors.
Numerous devices have been proposed, and in certain cases marketed, for insuring greater accuracy in the delivery of parenteral fluids. In some instances, such devices have simply indicated flow rates, leaving it to the medical technician or user to make manual adjustments of a standard flow-controlling valve, whereas in other instances such devices have been designed to automatically control flow as well as to indicate flow rate. In general, such efforts have been unsatisfactory because of complexity, inaccuracy, expense, and mechanical malfunctions in use. It is apparent the malfunctioning of a flow control device might have even more serious consequences for a patient than a miscalculation of dosage in the use of the simpler and more conventional drip chamber assemblies. Consequently, there is a need for a simple, accurate control device that is reliable and inexpensive to produce.
Accordingly, it is an object of the present invention to provide a system that shares the simplicity and other advantages of the drip chamber approach without the disadvantages inherent in such an approach. Specifically, it is an object to provide a device and method for converting a continuous fluid stream into a succession of discrete fluid segments traveling through a capillary passage, in contrast to free-falling droplets passing through a drip chamber, so that the partially confined segments may be observed and accurately measured. Since the capillary passage is of established cross section, measurement of fluid segment length and frequency provides all of the information needed for an accurate determination of flow rate.
An important aspect of this invention lies in the discovery that a continuous stream of fluid introduced into one end of an upstanding capillary tube will spontaneously separate into discrete segments with observable menisci if the capillary tube is provided with a longitudinal slit that places the bore of the tube in open communication with a chamber surrounding the tube and containing a second fluid immiscible with the first. The term "fluid" is used herein to refer to both liquids and gases. Ordinarily the first fluid will be a liquid introduced into the upper end of the capillary passage and the second fluid will be a gas such as air surrounding the segment generator; however, it is conceivable that the first fluid might be a gas and the second a liquid (in which case the direction of flow would be reversed), or that both fluids might be liquids (in which case the direction of flow would depend on their relative specific gravities).
The surrounding chamber may take the form of a drip chamber with the lower end of the slit capillary tube freely suspended or supported above the bottom of that chamber, and with the upper end of the capillary tube connected to any suitable conduit means for directing liquid to the upper end of the capillary passage. As the continuous column of liquid enters the capillary tube and reaches the top of the vertically-elongated slit, air segmentation of the liquid column spontaneously occurs with each liquid segment separating from the liquid column and accelerating until the gravitational, capillary, and frictional forces reach equilibrium. Subsequently, the segment or bolus travels at a constant velocity to the open lower end of the tube. The vertical slit permits umimpeded movement of each discrete liquid segment through the capillary passage because both the leading and trailing surfaces (i.e., menisci) of each segment are subjected to the same ambient pressure; hence, the slit not only initiates liquid segmentation but also promotes the unrestrained travel of such segments to the open lower end of the tube where they are discharged as droplets into the drip chamber.
The descending segments have their lateral surfaces defined by the capillary passage and, therefore, the flow of liquid through the passage may be precisely ascertained if the length and frequency of the segments are known. Detecting means along the tube may be used in conjunction with electronic measuring means for determining and displaying flow rate based on integrated segment length per unit time.
The capillary passage would ordinarily be straight and of uniform cross section. The wall of the capillary tube or generator is preferably transparent and its inner surface must be treated or formed so that it resists wetting by the liquid passing through it. To function effectively as a segment generator, the passage should be of capillary size, falling within the broad range of about 0.5 to 5.0 millimeters in diameter with a preferred range of about 1.0 to 3.0 millimeters. The longitudinal slit may have a width within the range of 0.2 to 1.5 millimeters, preferably 0.5 to 0.9 millimeters, and the length of the capillary passage should be at least one centimeter.
It has also been found that segment generation is enhanced if there is a lateral opening, preferably a lateral enlargement of the longitudinal slit, at or adjacent the upper end of the slit. Such an enlargement or cross slit is particularly helpful where relatively high flow rate capabilities are required.
Other features, advantages, and objects of the invention will become apparent from the specification and drawings.